In the field of optical systems wherein a lens is utilized for magnifying certain information, it is important that the lens is positioned and maintained in such position to provide proper focusing for reading the information. In the case of a projection-type lens, it is advisable to use a high-efficiency lamp along with a high-resolution lens to obtain optimum operating conditions.
Since proper focusing of the optical system is extremely important, various ways and means have been utilized for moving the lens along the axial direction in relation to an optical plane. One well-known method is to adjust the position of the lens along its optical axis by camming means which includes structure whereby the lens and the barrel are axially moved in relation to the optical or focal plane. Another method may use bevel surfaces in the nature of a helical curve whereby a pin or like member is caused to be moved along the curve for adjusting the lens in a manner to provide proper focusing thereof. In a number of these systems it is usually required to refocus the lens system upon reading of different information on microfilm or the like when such film is caused to be replaced in film viewing apparatus.
Additionally, an optical lens may be positioned and retained in a system which includes structure for securing the lens in a manner to avoid repeated re-focusing of the lens and such structure may be designed to make fast and simple changing of the lens which is then in the apparatus for a lens of a different magnification. Lens mounting systems have included bayonet-type structure but also use intricate machining to obtain a "spring effect" to hold a lens against a defined surface, as in the case of many of the lens mountings for cameras.
In the case of a viewer or reader for accommodating microfilm or microfiche it is well-known that the reduction ratio of such microform readers may vary, for example, from 24 to 150 times and also the ratio of the microfilm may vary. Because of these variations, it becomes advisable to have available a number of lenses of different magnification values rather than a plurality of readers with different reduction ratios.
The use of different magnification lenses in a specific reader or like apparatus presents a problem of maintaining focus when the lenses are interchanged to accommodate a microform or microfilm of like ratio. Additionally, it becomes necessary to provide structure which enables changing or replacing lenses in the shortest possible time.
Representative prior art of lens mounting includes U.S. Pat. No. 2,285,768, issued to A. Drucker on June 9, 1942, which discloses a lens mounting wherein a resilient element engages and holds an annular member against a friction-creating ball member and also engages and rests against a shoulder of a distal sleeve. In another form of the lens mounting, a resilient member engages one end of a sleeve to hold the sleeve against the end of a second sleeve.
U.S. Pat. No. 2,469,781 issued to J. M. Peters on May 10, 1949 discloses a lens holder and support which enables rapid changing of lenses and includes a threaded annular rib engaged by a barrel member, a lens assembly provided with a diaphram-changing ring and a flared skirt member secured to the ring.
U.S. Pat. No. 2,691,322 issued to O. Wittel on Oct. 12, 1954 shows a lens-mounting device with an inner ring and an outer ring along with a yieldable connecting member formed of resilient material and having a planar portion and an inclined or dished portion engaging the rings in a manner to retain the surfaces of the rings in engagement and to position the lens mount in axial direction.
U.S. Pat. No. 3,477,777 issued to A. Grosjean on Nov. 11, 1969 discloses a locking device for an adjustable optical element and includes an annular disc which clamps an adjusting ring against a wedge surface.